1. Field of Invention
The present invention pertains to novel stabilized slurry compositions and methods for making same, suitable for use in chemical-mechanical planarization processes, and more specifically to tungsten chemical-mechanical planarization processes. The invention has particular applicability to the semiconductor manufacturing industry.
2. Description of the Related Art
In the semiconductor device manufacturing industry, chemical-mechanical planarization (CMP) is used to planarize and reduce the contamination of structures overlying the semiconductor substrate. Most semiconductor manufacturers use CMP to planarize dielectric layers and metal structures. The most common approach used in the CMP process is to attach a semiconductor wafer to a carrier (which may or may not rotate) via a mounting pad and to polish the exposed surface of the wafer by bringing it into contact with a polishing pad. The mechanical abrasion between the wafer surface and the polishing pad results in the planarization of the wafer surface.
To aid in the planarization of the wafer surface and to transport disengaged wafer particles from the wafer surface, a slurry is usually introduced between the wafer surface and the polishing pad. Slurries typically include abrasive particles and a medium in which the abrasive particles are suspended. In addition, oxidizing agents are often blended with the slurry either at the point of use or on-site as per customer specifications. Surfactants can also be added to the slurry to enhance the wettability of the surface being polished and reduce vibrations during planarization. The chemical components of the slurry react with the wafer surface, thereby making the wafer more easily polishable.
Various processes used in the semiconductor manufacturing industry involve the formation of a layer of either a conductive or an insulating material followed by planarization of the surface of the material. For example, a dielectric layer, which is often quite thick in comparison to other structures, is typically blanket deposited over the entire wafer. Due to the underlying topology, the upper surface of this layer varies in height by a fairly appreciable amount. Using present lithography methods and equipment, the via holes/trenches that are to be patterned in this layer to form the conductive interconnects are difficult to fabricate because of the variance in the height of the upper surface. Hence, a planarization step is required prior to the patterning of these holes. Typically, this step is accomplished by CMP and results in a dielectric layer with a relatively planar upper surface. In addition, CMP can be utilized after the blanket formation of inlaid conductive structures such as vias and/or interconnects, such structures typically being made from tungsten (W), copper (Cu), aluminum (Al) or other metals or combinations of metals.
CMP more effectively planarizes structures and is cleaner than other planarization means, such as reactive ion etching (RIE). Typically, openings in the dielectric layer for the vias and/or interconnects are formed. Conductive material blanketly (or selectively) fills up the vias and/or interconnect openings. In order to properly fill these openings, an excess amount of the conductive material is preferably deposited over the wafer. This results in the formation of a layer of conductive material over the dielectric layer in regions other than the openings. In order to remove this excess material, a CMP process is performed.
Removal of conductive or dielectric material may be enhanced by introducing an oxidant to the CMP process so that the surface of the material becomes at least partially oxidized. Materials which are oxidized are often softer and more easily removable than the unoxidized material. For example, tungsten oxide can be more easily removed using CMP processes in comparison with unoxidized tungsten.
Semiconductor device manufacturers frequently add hydrogen peroxide (H2O2) or other oxidizing agents to slurries used in tungsten-planarization processes in order to oxidize portions of the tungsten structure (usually the peaks), thereby making them more easily removable by mechanical planarization. In such processes, hydrogen peroxide may be added separately onto the polishing pad. Alternatively and more preferably, hydrogen peroxide may be mixed into the slurry prior to the delivery of the slurry to the CMP tool. Prior mixing of the hydrogen peroxide increases control and operational simplicity. See John P. Bare and Budge Johl, Evaluation of Manufacturing Handling Characteristics of Hydrogen Peroxide-Based Tungsten CMP Slurry, IEEE/CPMT International Electronics Manufacturing Technology Symposium, 164-71 (1998).
A problem with using oxidizing agents in slurries is the relatively rapid decomposition of the oxidizing agent while in the slurry. For example, hydrogen peroxide-containing slurries have a relatively short lifetime. A common observation in the industry is that the hydrogen peroxide concentration in slurries decreases at a rate of about 0.1% per day in both static baths and distribution systems. A typical useful lifetime of a hydrogen peroxide-containing slurry with an initial hydrogen peroxide concentration of 2% is about 6 to 7 days. The useful lifetime is reduced to about 2 days for slurries with an initial hydrogen peroxide concentration of 4%. These slurries thus have limited useful lifetimes.
The decomposition of hydrogen peroxide in slurries creates process-related problems. For instance, because CMP process tools are designed to operate for a specific concentration of hydrogen peroxide in the slurry, any reduction in hydrogen peroxide concentration adversely affects quality and throughput. In addition, the decomposition of hydrogen peroxide reduces the shelf life of a hydrogen peroxide-containing slurry, which may create complications in the transport and storage of the slurry.
FIG. 1 is a graph of hydrogen peroxide concentration versus time, and illustrates the decomposition of hydrogen peroxide in a typical commercially available tungsten slurry mixture. A result of this decomposition of hydrogen peroxide is that the tungsten removal rate for a CMP process utilizing the slurry decreases at a rate which is not completely predictable. The removal rate eventually decreases to a point where the slurry is no longer usable. For example, tungsten CMP slurry compositions containing an initial hydrogen peroxide concentration of about 2% and 4% by weight become unusable when the concentration decreases to less than about 1.5% and 3.8% by weight, respectively.
The decomposition of hydrogen peroxide mixed in a slurry can be attributed to several different factors. UV radiation impinging upon the slurry and hydrogen peroxide mixture, elevated temperatures and the high-pH of some slurries have been recognized by many in the industry as major factors contributing to the decomposition of hydrogen peroxide. For example, for each 10xc2x0 C. incremental increase in temperature, the hydrogen peroxide rate of decomposition may increase by a factor of about 2.2. Increase in the rate of decomposition may also occur at pH levels greater than about 6 to 8.
The presence of heavy metals in the slurry may be the controlling cause of decomposition of hydrogen peroxide in slurries. In this regard, slurries can contain trace amounts of heavy metals (e.g., iron (Fe), copper (Cu), cobalt (Co), manganese (Mn), nickel (Ni) and chromium (Cr)). For example, a typical tungsten planarization slurry (W2000 manufactured by Cabot Corp.) can contain as much as 75 ppm iron, 2 ppm chromium, 1 ppm copper, 1 ppm cobalt, 1 ppm magnesium and 1 ppm manganese. These heavy metals are capable of catalytically accelerating the decomposition of hydrogen peroxide in the slurry, thereby decreasing the degree of oxidation and planarization of the surface of the substrate being treated.
In response to this problem of hydrogen peroxide decomposition, the use of chelating agents capable of complexing the heavy metals in solution to decelerate the decomposition of hydrogen peroxide has been proposed. Polyphosphoric and phosphonic acids and salts thereof have been used to stabilize hydrogen peroxide in non-slurry environments. See, e.g., U.S. Pat. No. 5,290,361 and U.S. Pat. No. 5,302,311. Other chelating agents such as ethylenediaminetetraacetic acid (EDTA), 8-hydroxyquinoline and Na-EDTA have also been used to stabilize hydrogen peroxide.
However, not all chelating agents work effectively in the chemical environment of a slurry mixture. FIG. 2 is a graph of hydrogen peroxide concentration versus time for a commercial CMP slurry to which unstabilized hydrogen peroxide has been added and for a stabilized hydrogen peroxide (PC Grade from Solvay) added to the same slurry. The CMP slurry containing the stabilized hydrogen peroxide did not exhibit a drastic improvement over the slurry containing the unstabilized hydrogen peroxide. While the actual composition of the commercial stabilized hydrogen peroxide solution is proprietary to the manufacturer, the stabilizing agents are believed to include potassium stannate, tetrapotassium pyrophosphate and phosphoric acid, each present in a concentration less than 2% by weight.
FIG. 3 is a graph of hydrogen peroxide concentration versus time for hydrogen peroxide-containing tungsten CMP slurries which contain no stabilizer, 0.3 g EDTA, 1.0 g EDTA and 1.0 g phosphonic acid. From this graph, it can be seen that the decomposition of hydrogen peroxide actually increased with the addition of these stabilizers to hydrogen peroxide in tungsten CMP slurries.
FIG. 4 is a graph of hydrogen peroxide concentration versus time for hydrogen peroxide-containing tungsten CMP slurries which contained no stabilizer, Na-EDTA, Na-pyrophosphate and 8-hydroxyquinoline. From this data, it can be seen that the decomposition of hydrogen peroxide in tungsten CMP slurries also increased with the addition of Na-EDTA and 8-hydroxyquinoline.
In view of the foregoing, there is a need in the art for a stabilized slurry composition which can effectively decrease the rate of decomposition of peroxygen compounds added thereto, particularly hydrogen peroxide.
To meet the requirements of the semiconductor manufacturing industry and to overcome the disadvantages of the related art, it is an object of the present invention to provide novel stabilized slurry compositions suitable for use in chemical-mechanical planarization processes, for example, in the chemical-mechanical planarization of a tungsten-coated semiconductor wafer. In particular, it is an object of the invention to provide a slurry composition which includes a stabilizing agent which decreases the rate of decomposition of the peroxygen compound present in the slurry composition.
Other objects and aspects of the present invention will become apparent to one of ordinary skill in the art upon review of the specification, drawings and claims appended hereto.
The foregoing objectives are met by the compositions of the present invention. According to a first aspect of the invention, a slurry composition suitable for use in a chemical-mechanical planarization process is provided. The composition comprises:
(a) abrasive particles dispersed in a suspension medium;
(b) a peroxygen compound; and
(c) a stabilizing agent selected from the group consisting of a phosphoric acid, a salt of a phosphoric acid and combinations thereof.
In accordance with a further aspect of the invention, a slurry composition suitable for use in a chemical-mechanical planarization process is provided. The composition comprises:
(a) abrasive particles dispersed in a suspension medium;
(b) hydrogen peroxide; and
(c) sodium pyrophosphate decahydrate.
In accordance with a further aspect of the invention, a slurry composition suitable for use in a tungsten chemical-mechanical planarization process is provided. The composition comprises:
(a) abrasive particles homogeneously dispersed in a suspension medium, wherein the suspension medium comprises acidified deionized water;
(b) hydrogen peroxide present in an amount greater than about 0.5% by weight of the slurry composition;
(c) sodium pyrophosphate decahydrate present in an amount from about 0.025% to about 0.3% by weight of the slurry composition; and
(d) a surfactant.
In accordance with yet a further aspect of the invention, a method for manufacturing a slurry composition suitable for use in a chemical-mechanical planarization process is provided. The method comprises the steps of:
(a) providing a slurry comprising abrasive particles dispersed in a suspension medium;
(b) adding a peroxygen compound to the slurry; and
(c) adding a stabilizing agent to the slurry, wherein the stabilizing agent is of a material selected from the group consisting of a phosphoric acid, a salt of a phosphoric acid and combinations thereof.